In the fluorescent lighting industry, it is common practice to use a ballast, typically an inductor or a capacitor, to control the flow of alternating current. High-frequency fluorescent lighting systems, however, utilize an electronic ballast circuit which includes a ballast together with an inverter for generating a high frequency electric current which supplies virtually all of the power needed to operate the lamps. Also, a filament transformer is sometimes included in the electronic ballast circuit to heat the filaments of the lamps.
Existing electronic ballast circuits controlling high-frequency lamps have a tendency to overheat and fail when the lamps near the end of their lives. The gas discharge lamps operate on alternating current (AC) with the current flowing through the lamps in both directions, between two electrodes that act alternately as the cathode and the anode, and vice versa. Frequently, as the lamps become older one of the two electrodes loses its ability to emit electrons while the other electrode does not. When this occurs, the lamp conducts electric current more readily in one direction than the other. Thus, the lamp assumes the characteristics of a rectifier and generates direct current (DC) from the alternating current normally supplied to it. The direct current can burn out the primary winding of the filament transformer if that winding is connected to the rectifying lamp in such a way that direct current can flow through a closed path that includes the lamp and the filament transformer. This direct current is produced by the lamp and not by the high-frequency generator or inverter which furnishes AC power to operate the lamp, ballast and filament transformer combination.
This problem may cause significant additional harm in electronic ballast circuits that utilize an inductive ballast. An inductive ballast cannot block the flow of direct current. Thus, even a small DC current tends to saturate the magnetic core of the ballast. This reduces the inductance of the ballast and, therefore, interferes with its ability to limit or control the flow of alternating current. Thus, not only the magnitude of DC current but also the AC current can increase to a level which may damage the ballast inductor and other components in the inverter.
This is an unrecognized problem, especially in the case of high-frequency fluorescent lamps systems. Existing prior art systems, such as Paget U.S. Pat. No. 3,579,026 (FIG. 1), Zaderej U.S. Pat. No. 4,042,852, Spira U.S. Pat. No. 4,027,498, Capewell U.S. Pat. Nos. 4,207,497 and 4,210,846 (FIG. 4), Josephson U.S. Pat. Nos. 4,388,562 (FIG. 1) and 4,513,226, and Petersen U.S. Pat. No. 4,349,863 (FIG. 2) show designs in the indicated figures which are susceptible to damage as a result of excessive direct current.
In particular, with respect to low-frequency applications, ballast inductors have a relatively high winding resistance which is more effective in limiting the flow of direct current. Thus, low frequency ballasts heat up relatively slowly, enabling an over-temperature thermostat which is typically incorporated within the ballast, to turn off the power before any significant damage to the ballast occurs.
High-frequency ballast inductors, on the other hand, have a low winding resistance since a lower inductance is sufficient to control lamp current. In some cases, larger diameter wire is utilized to reduce electrical resistance and maximize efficiency. Due to this reduced resistance, the direct current resulting from the rectifying lamp can be much greater in high-frequency systems than in more familiar systems. This damages the ballast inductor or the inverter quickly before the protective thermostat has time to respond. For example, in some cases direct current through the lamp and ballast has been measured at four amperes, or about 10 times the normal lamp current.
Electronic ballast circuits for rapid-start fluorescent lighting applications sometimes include a filament transformer for heating the cathodes of the lamps. The voltage across the primary of the filament transformer is obtained directly from the high voltage across one or more lamps. The high-voltage primary winding of the filament transformer is therefore easily damaged by direct current from a rectifying lamp.
Presently available ballasts utilizing capacitors for the current limiting function do not conduct direct current. However, even though the ballast capacitors prevent damage to themselves and to the inverter components from rectifying lamps, they do not protect the filament transformers.